Depth of Field Preview
Some cameras have a function to preview the depth of field. To understand how it works, you first need to know what the depth of field is, for example, you can read about it here.
Modern cameras always perform focusing at the maximum aperture. For example, if a standard 18-55mm F / 3.5-5.6 lens is used, then focusing will always be done on apertures from F / 3.5 to F / 5.6 (depending on the focal length). But when shooting (shutter release) such a lens can use a very wide range of aperture values, for example, a Canon Zoom Lens EF-S 18-55mm 1: 3.5-5.6 IS can shoot at apertures ranging from F / 3.5 – 5.6 to F /22.0 – 36.0. In this case, when focusing, it is impossible to see in the optical viewfinder how the image obtained when shooting at closed apertures from F / 5.6 to F / 36.0 will look. When the diaphragm is closed (increasing the F number), the depth of field changes. Depth of field preview exists to see how much the depth of field will change when shooting at a closed aperture.
Depth-of-field preview is a useful option when taking macro shots and shooting with high-aperture lenses, where a change in the F number entails a serious change in the depth of field. The easiest way to understand is how the depth of field preview option works in camera mode ‘Aperture Priority (A, AV)‘. It is enough to use this function for different values of F and observe what is happening in the optical viewfinder (camera eye). Usually, camera users notice only darkening of the viewfinder – this is due to the fact that at the closed apertures in the HIR there is less light and the subject appears darker. The darkening will not appear in any way on the image that will be performed with the aperture closed, as the camera automatics will select the correct exposure for the small aperture. But besides the darkening it is worth noting that in the JVI there are more and more ‘sharp’ and ‘clear’ objects.
On some cameras to preview the depth of field a special button can be highlighted. It is usually called ‘DOF preivew’. When you click on such a button, the camera automatically sets the specified aperture in the settings. True, there are certain limitations, for example, my Nikon D700 camera refuses to function as a ‘DOF preivew’ button when Live View is turned on. On old and modern film cameras, such a button is sometimes called the Ди Aperture Repeater ’, since such a button‘ rehearses ’the SIPT before the main shooting, because you cannot immediately see the finished result on the film cameras. The aperture repeater is on some old lenses, for example on the Olympus OM-System G.Zuiko Auto-W 1: 3.5 f = 28mm. On lenses that have an aperture control ring and / or an aperture preset ring, you can do a preview of the depth of field manually using the aperture ring and / or using the preset ring.
I do not use such a preview button. From personal experience, I always know by how much the depth of field will increase when the diaphragm is closed. And even if I need to look at what happens, I just take a test shot at the desired aperture value and get the finished result immediately. I really do not like that on many cameras, for example, on the Nikon D80, the ‘DOF preivew’ button is not programmable. I usually set a more useful feature on the ‘DOF preivew’ button. So, on the Nikon D700, the ‘DOF preivew’ button on me serves to adjust the auto ISO or to block the flash triggering. Many users believe that the ‘DOF preivew’ button on advanced cameras will somehow help improve their photos, but this happens very rarely. Therefore, when choosing a camera, you should not pay attention to the function of viewing the depth of field. By the way, if the camera does not have such a function, then the GRIP can be viewed very easily with the help of Live View (though not for all cameras), or simply take a test shot.